Data transmitting and receiving equipment, such as the standard telephone, have been commonly interfaced to the commercial telephone lines using a 600 ohm transformer and a relay to electrically isolate the equipment from the lines. Although generally satisfactory for voice signals, transformer coupling is unsuited for high bit rate data transmission because the high frequency response of the transformer is limited by interwire capacitance. Furthermore, the core of the transformer must be quite large, on the order of several inches square, to prevent saturation under the relatively high DC current that will pass through the transformer coil from the 48 volt source on the telephone lines. The relay, which is often packaged together with the transformer, further increases the size of the interface and reduces its overall reliability because the relay contacts tend to wear and corrode with time and there are problems with vibration and contact bouncing during closing.
In order to eliminate the transformer and relay, electrical isolation from the telephone lines has more recently been provided by acoustic coupling, wherein electrical signals representative of voice or binary data are converted to high frequency sound waves in a transducer and acoustically coupled to the telephone transmitter. Incoming corresponding signals on the telephone lines are converted to acoustic signals at the interface and acoustically coupled to the receiver.
Whereas transformer or acoustic coupling has a frequency response that is generally sufficient for voice communications, each is insufficient for interfacing with telephone lines modems (modulator-demodulator units) that transmit and receive digital signals and enable communication between computers on the telephone lines. The modem carrier frequencies, even for high speed modems, is still within the voice band 300 Hz-3 KHz. The switched network simply will not pass higher or lower frequency components.
Accordingly, one object of the present invention is to provide a new and improved optical interface for electrically isolating equipment from a set of communication lines.
Another object is to provide a telephone line isolation circuit that has improved high frequency phase and amplitude response as well as improved linearity.
Another object is to provide a new and improved telephone line isolation circuit that uses solid state components for increased reliability and negative feedback for improved stability.
In order to increase the high frequency response of telephone line isolation couplers, it has been proposed to use light emitting diode (LED) and light responsive transistor (LRT) pairs as the isolation elements for interfacing the modem and telephone lines. Presently available telephone line optical isolators, however, exhibit strong non-linear characteristics which present problems in telephone line interfacing in view of strict specifications promulgated by the telephone companies. U.S. Pat. No. 4,056,719 to Waaben, for example, discloses an optical coupler for interfacing equipment with the telephone lines that provides separate optical coupling paths for transmission and reception. Optical coupling of data along each of the optical paths is provided by an LED/LRT pair, wherein each LED is driven by an associated driver amplifier responsive to electrical signals generated by the modem or to incoming electrical signals on the lines. Optical feedback from each LED to the input of its associated amplifier helps improve the linearity of the LED, although the overall stability and frequency response of the interface are not substantially improved because the data and feedback paths are separate; negative feedback for bias stabilization is limited to only the individual LEDs and associated driver amplifiers and does not intersect the LRTs and associated circuitry of the interface. In practice, however, to maintain a high degree of stability, the data and feedback optical paths must be matched precisely and this requires matching the LEDs and LRTs associated with each incoming or outgoing signal path. Because there is negative feedback only between the input and output of each LED driver amplifier, the transfer characteristics of the optical devices, which tend to change as a function of temperature and aging, degrade the long term stability of the interface.
Another object of the invention, therefore, is to provide an optical isolation circuit for interfacing equipment and the telephone lines, wherein variations in component characteristics as a function of temperature, aging or other factors are compensated.
Another object is to provide a new and improved optical isolation circuit for interfacing data transceiving equipment and the telephone lines, wherein data and feedback are both optically coupled along common optical paths for improved circuit stability.
Another object is to provide a new and improved optical isolation circuit for interfacing data transceiving equipment and the telephone lines which satisfies commercial telephone line impedance characteristics independently of telephone line voltage or line impedance.